Even after the PQ is over and the winning side is decided, the losing side's leader can disband his own party and cause everyone to be moved from the PQ map. This can prevent the winning side from picking up their gift. This is very frustrating.
The button is disabled in the Balloon PQ, and /leaveparty shows an error. Are you sure they are disbanding the party? Looked into this, cannot see any way for this to happen. My best guess is that your team actually lost, got warped out correctly after 3 seconds, and had your party disbanded automatically as intended.
@Matt Glad to know that this option is disabled. IDK how but there was this one time that my team won and we were warped out before I could pick up my gift.